1. Field of the Invention
This invention relates to the field of X-ray machines. More specifically, the invention comprises a conical anode X-ray source with an aligned homogenizing optic.
2. Background of the Invention
FIG. 1 shows a traditional X-ray generating device. Electron beam 40 is focused on reflecting surface 42 of rotating anode 44. X-rays are generated by the electron beam striking the surface. These reflect laterally as X-ray beam 46, which are emitted from the machine through side window 48. The prior art anode is rotated to prevent overheating, since the electron beam would cause local overheating if it remained focused on one spot of the anode.
Although the present invention has applications in many fields, it is particularly well-suited to the field of mammography. Accordingly, the reader may wish to know some of the constraints particular to this field. Mammography requires very high resolution radiographs in order to provide the details needed for diagnostic work. However, because a-patient typically undergoes screening mammograms on numerous occasions, it is important to keep the dosage as low as possible.
Spatial resolution requirements suggest that a small source of X-ray emissions would be best. However, it is difficult to make and X-ray source that is sufficiently small yet powerful enough to provide ultra-high resolution in the resulting image. Known sources also produce too much high-energy radiation, which produces background contrast problems. The apparent spot size is also not constant across the image plane. The spot size also tends to change over the time so the instruments must frequently be recalibrated.
Radiography of soft tissue is normally done with lower X-ray energy than dense tissue or bone. The preferred energy range for mammography is about 17-25 KeV. The soft tissue features show better contrast using low energy X-rays. Higher energy (“harder”) X-rays simply produce more background due to scatter. The potential used to produce the desired molybdenum “K” line of X-rays is about 17.5 KV X-ray tube potential. The production of X-ray lines is proportional to the overvoltage of the X-ray tube, according to the following expression:
IK∝i(V−VK), where IK is the intensity of the characteristic molybdenum K line, i is the tube current, VK is the minimum potential necessary to produce the Mo(K) line (17.5 KV), and V is the actual tube potential.
Tubes for mammography are normally operated at about 26 KV to minimize the high energy Bremstrahlung (“braking radiation”) that causes background problems. A molybdenum filter is sometimes used to minimize the high energy Bremstrahlung while still passing the desired Mo(K) line, but this also suppresses some of the desired emissions. If the production of Bremstrahlung could be minimized, then the electrical potential of the tube could be raised and the intensity of the desired line radiation could thereby be increased.
For example, if VK is 17.5 KV, once may easily see from the expression IK∝i(V−VK) that raising V from 26 KV to 50 KV would increase the intensity of the desired line radiation by a factor of about 9. Ordinarily this increase would produce a substantial increase in Bremstrahlung, largely if not completely eliminating any gain in image resolution. The reader will therefore appreciate that an X-ray optic able to increase the intensity of the desired line radiation without producing a related increase in Bremstrahlung, would be highly desirable. The present invention proposes just such a device. The present invention also substantially improves the uniformity of the desired emissions.